1. Field of the Invention
The present invention relates to a tufting machine used for carpet manufacture.
2. Description of the Related Art
Tufting machines are distinguished over other carpet making methods in that loops of yarn which constitute the pile of the carpet are inserted in a backing medium or cloth, which may be fibrous or woven according to the carpet application. The loops are held in place by the retentive pressure of the backing cloth having been expanded locally through the insertion of the yarn. A subsequent operation covers the rear face of the yarn and backing cloth with a retaining adhesive. The adhesive also holds a further layer of backing material, usually hessian.
The yarn is inserted into the backing material by a multiplicity of needles which perform a reciprocating motion. The needles have eyes at the lower extremities through which yarn is both fed and captured. Generally, the needles are connected to one or more transverse bars known as needle bars so that all needles may be reciprocated together into and out of the backing material. In certain specialized tufting machines, the needles are carried by needle holders that may be selectively latched to a reciprocating latch bar so that the needles are capable of individual selection and only those needles selected are subject to the reciprocation, whilst those not selected are not reciprocated. The reciprocating action usually is delivered through a series of pistons or push rods couple by various conventional means to a rotating main-shaft driven by electric motor or similar means. The coupling mechanism is of a crank-shaft type so that the extent of the needle motion is the throw of the crank. Other more complex arrangements are also known which endow features to enable the motion envelope of the needle to be controlled and easily set to any desired range.
The loops of yarn may be of varying heights on the face side of the carpet in order to provide a patterning effect. There are several techniques for causing this effect, such as changing the tension of yarn from low to high from one insertion of a yarn loop to the next (high pile to low pile). Yarn tension may be set by modulating the speed of the yarn feed mechanism controlling the length of yarn delivered for each loop (stitch). A simple form of yarn feed mechanism uses a pair of rollers with a high friction surface between which the yarn is pinched. Variations in the speed of the rollers allows control of the length of yarn delivered for each tuft of carpet pile. There are many more complex mechanisms for exerting control over the yarn delivery. Some of these deliver control over individual strands of yarn, some over subsets of all the strands.
Additional patterning features include the use of cut pile as opposed to pile formed from loops. The cut pile effect is achieved usually during the tufting process by catching the loops formed by the insertion of yarn through the backing cloth on a suitably formed hook on the face (lower) side of the carpet. A knife shears the yarn on one side of the hook after several further loops have been inserted in the backing material. The knife is articulated to move in synchronism with the insertion of yarn. The hooks involved in the loop capture during yarn insertion are also arranged to move into position in synchronism with the yarn insertion process.
The foregoing is a non-exhaustive illustration of some of the features of tufting machines and the mechanisms for controlling carpet patterning. Further features of tufting machines include the ability to introduce a plurality of colors. Tufts of different colored yarns can be made to form attractive carpet patterns such as can be achieved in woven carpets in which any chosen color may be inserted in any location in the carpet by correct design of the patterning commands. One way in which this is done is by burying one colored yarn by giving it a very low pile height in an area of higher pile height of different color by the well known process of backrobbing yarn from the previous stitch to reduce the pile height of that previous stitch. When the buried color is required, the pile height is set high whilst that of the other color in that area is set lower. This results in patches of different colors but has a number of disadvantages in the wastage of yarn which cannot be seen and in the straight line arrangement of the colored yarn. Means for altering the lateral position of the yarns have consequently been developed to overcome some of these limitations.
There are normally up to two needle bars on a machine although more may be fitted. These are reciprocated by the main-shaft rotation and crank means in the vertical direction. An additional degree of freedom of motion is afforded to the needle bars by a mechanism which allows side to side motion, that is across the width of the tufting machine laterally. Yarns may thus be moved from one needle position to others. This provides greater flexibility to produce a required pattern, as needles containing a particular color can be shifted laterally to a position where that particular color is required in the pattern.
Mechanisms for moving the needle bars include cam driven systems, hydraulic actuators, pneumatic actuators, and electric servo controlled motors with a rotational to linear conversion device. This latter mechanism is typified by WO 97/15708 and EP 867,553. In these cases, the rotational to linear conversion devices include screw thread and nut arrangements (including those with acme threads and other thread profiles; ball screws; inverted roller screws and similar equivalent devices). These mechanisms provide the necessary motional requirements from a functional view but also limit the speed capability of the tufting machine. This limitation may arise due to the speed, rate of acceleration and slackness or free play in the mechanisms connecting the servo motor with the sliding needle bar; other relate to the amount of force required to provide the acceleration, inertia effects and control loop stability.
Further, with these systems, the sliding needle bar drive system, servo motors, or equivalent hydraulic or pneumatic actuators have been mounted on the external and faces of the tufting machine. This has required the provision of holes in the end plates or housing to allow the servo motor output shaft to feed into the sliding needle bar assembly. The servo motor output shaft has been coupled to a ball screw or equivalent device (such as an inverted roller screw or screw and nut assembly). The motion sensor for the servo motor has usually been coupled directly to its rotating shaft so that lost motion in any coupling mechanism between the servo and the sliding needle bar has remained uncompensated.